The photon-driven (2.5 to 6.4 eV) desorption of SO2 (coverages up to 2.7 monolayers) on Ag(111) has been studied by time-of-flight mass spectrometry in combination with temperature programmed desorption and Auger electron spectroscopy. The photodesorption yields per incident photon increase with photon energy. Consistent with substrate-mediated excitation, the mean translational energy [E(trans)/2k] is constant (650 K) for photon energies between 3.5 and 6.4 eV, but decreases at longer wavelengths (480 K at 2.5 eV). The decrease is attributed to changes in the energy distribution of the hot substrate carriers responsible for desorption. The photodesorption yields vary with the initial coverage, the method of preparing the initial coverage, and the extent of photolysis. These variations are attributed, in part, to changes in the SO2 orientation on the surface. Even at 6.4 eV, where unimolecular photodissociation occurs in the gas phase and multilayer, there is no photodissociation in monolayers. This effect is attributed to strong substrate quenching.